1. Field of the Invention
This invention relates generally to recovery of a metal ion from an aqueous solution and in particular to removal of copper ions and other metal ions from rinses associated with electroless (chelated) plating baths.
2. Description of the Prior Art
Electroless plating solutions typically contain salts of metals such as cobalt, copper or nickel along with a reducing agent such as sodium hypophosphite, complexing agents such as ethylenediaminetetraacetic acid (EDTA), a pH adjuster such as sodium or ammonium hydroxide and possible stabilizers such as cyanide or thiourea. The electroless plating baths contain high concentrations of chelated metal ions and are typically operated at high pH values because the chelating compounds used in the bath chemistry hold more metal ions in solution at high pH values than at low pH values.
In electroless plating, a part, such as a printed circuit board, is placed in the electroless plating bath for a period of time and after the plating process is complete, the part is usually rinsed in a flowing stream of deionized water. The rinse water is collected for treatment because the water contains complexed metal ions, usually in the form of anionic complexes, and therefore cannot be discharged directly to a drain. The use of anion exchange resins to effect the removal of the anionically charged metal ions from the rinse water is complicated by the presence of high concentrations of other anions, such as hydroxide, cyanide, complexing agents, phosphites, phosphates, carbonate and sulfate, which are typically found in the rinse water.
Conventional hydroxide precipitation systems, which operate at high pH, generally are not effective in removing the metal ions from the rinse water because the chelating agents used in electroless plating form soluble metal ion complexes, which are very stable at high pH. Alternative forms of rinse water treatments such as borohydride reduction or the use of dithiocarbamates require high chemical dosage with extremely expensive reagents. Previous attempts to use ion exchange technology in the removal of EDTA chelated metal ions, such as copper ions, from electroless plating bath rinses have been unsuccessful.
The metal ion complexes in the plating baths are usually anionic. Therefore conventional cation exchange resins are not useful for extracting the metal ions from the rinse water. Conventional anion resins would seemingly be useful but, in fact, the rinse water contains overwhelming amounts of competing anions which rapidly reduce the capacity of the anion resins to unacceptable levels. Chelating ion exchange resins have had some limited success but most chelating agents used in electroless plating baths have such high affinities for the metal ion of interest that they inhibit metal binding by the chelating ion exchange resin. As a result the binding capacities of the chelating ion exchange resins are too low to be practicable.
Thus, ion exchange resins are relatively ineffective for recovery of the metal ions from the electroless plating bath rinse water. Nevertheless, if a method could be found to use conventional ion exchange technology for the removal of complexed metal ions from the rinse water, the method would provide significant advantages over the present systems for recovery of the metal ions.